It is known that uranium occurs principally in oxide form in its various ores. Furthermore, uranium usually occurs in relatively lean ores averaging, for example, approximately 0.3% uranium content. Uranium ores are for the most part not amenable to concentration by conventional means. Accordingly, the ore as extracted from the ground is simply crushed, ground and leached to yield a uranium containing solution and a barren rock waste which is rejected.
Processes which have been used for leaching uranium involve either acidic or basic leaching means. The common means employed in treating most North American lean uranium ores involves leaching with sulfuric acid along with an oxidizing agent which may be, for example, manganese dioxide, oxygen or sodium chlorate. In order for leaching with sulfuric acid to be successful, the ore being treated must also contain iron. If the ore contains insufficient iron, iron may be added, as metallic iron. Ferric iron plays an important role in the oxidation of tetravalent uranium. It may be considered that the acid leaching of uranium from ores containing uranium in the +IV oxidation state proceeds according to two steps: EQU UO.sub.2 +4H.sup.+ .fwdarw.U.sup.4+ +2H.sub.2 O EQU U.sup.4+ +2Fe.sup.3+ +2H.sub.2 O.fwdarw.UO.sub.2.sup.2+ +2Fe.sup.2+ +4H.sup.+
These reactions proceed simultaneously and the overall reaction can be given as: EQU UO.sub.2 +2Fe.sup.3+ .fwdarw.UO.sub.2.sup.2+ +2Fe.sup.2+
Conventionally, oxidants such as sodium chlorate or manganese dioxide are used to oxidize ferrous iron to ferric iron. The ratio of ferric to ferrous ions in the solution determines the oxidation potential thereof.
Although no mention of uranium is present therein it is known from U.S. Pat. No. 2,816,819 that iron in a solution which also contains nickel or cobalt can be oxidized from the ferrous to the ferric state by introduction of a mixture of sulfur dioxide and air thereinto.
Again, it is known from U.S. Pat. No. 3,869,360 that sea nodules can be treated with sulfur dioxide in the presence of oxygen to form the water soluble sulfates of manganese, nickel, copper and cobalt. It is considered in this patent, however, that sulfur dioxide acts as a reducing agent with respect to the metal value content of the sea nodules and it is stated therein that soluble iron sulfate formed is converted to the insoluble oxide.
In the Panel Proceedings Series, Uranium Ore Processing of the International Atomic Energy Agency, Vienna, 1976 p. 32 under the heading "Ferric Leaching and Autoxidation of Recycled Solutions", it is stated that a process wherein air and sulfur dioxide were blown into solutions containing ferrous sulfate to form sulfuric acid and ferric sulfate was studied during the early stages of the development of the acid leaching process for uranium extraction from Witwatersrand cyanide residues.
An article entitled "Leaching of High-Solids, Attritor-Ground Chalcopyrite Concentrate by in situ Generated Ferric Sulfate Solution" in Metallurgical Transactions B Vol 11B, March 1980, describes leaching of copper from chalcopyrite pulps containing up to 20% solids using a mixture of oxygen and sulfur dioxide introduced into the pulps to oxidize ferrous ion to ferric ion for leaching copper from the sulfide.
It is known that once the uranium values of an ore are dissolved as sulfate, uranium can be recovered by ion exchange or solvent extraction.